This invention relates to an integrated circuit. In particular, this invention relates to an integrated including a gas sensor. The invention further relates to a method of making such an integrated circuit.
Nowadays, integrated circuits may comprise a plethora of different sensors, such as gas sensors, relative humidity (RH) sensors, specific analyte detection sensors, and so on.
Gas sensors are used in a number of different applications to sense the composition and/or concentration of various gases. One example application is in the field of supply chain monitoring, in which the levels of CO2 present in the air surrounding consumables such as food or beverages is monitored to determine suitability for consumption. The monitoring may typically be carried out at various stages in the distribution chain. Other applications include air quality monitoring, use in heating, ventilation and air conditioning (HVAC) system in buildings or automobiles, or CO2 monitoring in greenhouses.
It is particularly relevant to mass market applications such as RF tags for product monitoring that the gas sensor functionality can be added to the integrated circuit with limited additional cost, as there is a large price pressure on such integrated circuits; i.e. they have to be produced cheaply in order to be commercially attractive.
FIG. 1 illustrates an example of an integrated circuit including a gas sensor. The integrated circuit includes a substrate 2 into which may be incorporated a number of active components such as CMOS devices. As is well known in the field of integrated circuit manufacture, above the substrate there is be provided a metallization stack 4 incorporating a plurality of metallic layers separated by a plurality of insulating layers. The metal layers provide interconnections between the active components in the substrate 2, and typically comprise metals such as aluminium or copper.
In this example, the gas sensor is provided above the metallization stack 4. In particular, the gas sensor is located above a series of passivation layers 16A, 16B and 18, which are themselves conventionally located on top of the stack 4. In the present example, layers 16A and 16B comprise High Density Plasma (HDP) oxide, while the layer 18, which provides scratch protection, comprises a thick layer of Si3N4. As shown in FIG. 1, metal vias 8 pass through the passivation layers to connect electrodes 15 of the gas sensor to the metallization stack 4. This allows electrical connection to be made between the gas sensor and one or more of the active components in the substrate 2 via the metallization stack 4. The vias 8 and electrodes 15 may comprise the same material as the metal layers in the metallization stack 4, or can alternatively comprise a different material, such as Tungsten.
Also shown in FIG. 1 is a protection layer 14, comprising for example Ta2O5, which provides protection against corrosion of the electrodes 15.
Above the protection layer 14 there is provided a thick oxide layer 17. Through the oxide layer 17, protection layer 14, and the passivation layers 16A, 16B and 18, there is provided a trench 20 at the bottom of which is provided a bond pad 12 in an upper metallization layer of the metallization stack 4. The trench 20 thus enables electrical connections to be made to the integrated circuit through the various insulating upper layers.
The gas sensor itself comprises a sensor element 8, which is shown in cross section in FIG. 1. The sensor element 8 typically comprises a metallic material, for example Tungsten. In an alternative embodiment, the sensor element 8 may comprise a semiconducting material such as doped polysilicon. The sensor element 8 may be arranged in, for example, a meander pattern for increased surface area (resulting in greater sensitivity). As shown in FIG. 1, the ends of the meander pattern pass through the protection layer 14 to connect to the sensor electrodes 15. Also as shown in FIG. 1, the meander pattern itself is located substantially within a shallow trench formed in the oxide layer 17. The sensor element 8 is thus presented to the surrounding environment for direct access to a gas to be sensed.
Thus, FIG. 1 constitutes an example of a gas sensor provided in an integrated circuit above the passivation stack on a metallization stack which is itself provided above active components such as CMOS transistors in a semiconductor substrate 2.
The gas sensor is thermal conductivity based, and operates as follows. A current is passed through the sensor element 8, causing the sensor element 8 to heat up. The surrounding gas carries heat away from the sensor element 8. The amount of heat that is transferred, and the rate at which it is transferred, is dependent upon the composition of the gas. At thermal equilibrium, the resistivity of the sensor element 8 (which is dependent upon the temperature of the sensor element 8) is sensitive to the amount and rate of heat transfer. In turn therefore, the resistivity of the sensor element 8 is dependent upon the composition of the surrounding gas. In this way, by making resistivity measurements of the sensor element 8, the composition of the surrounding gas can be determined.